ELLICOTT CITY, Md. (WJZ/AP)—- Investigators are still trying to answer questions about how a CSX train went off the tracks in Ellicott City, killing two teenagers. Both the cleanup and investigation are a painstaking process.

Meghan McCorkell has the latest.

Federal investigators say they are leaving no stone unturned in this investigation, though they say it will be a long process.

As the cleanup continues, questions still linger about what caused the deadly train derailment.

“That’s what we’re here to find out– not just what happened, but why it happened,” Jim Southworth, a National Transportation Safety Board (NTSB) investigator, said.

Piece by piece, the track where the train derailed is taken apart. Investigators will put it back together to see if it provides any clues.

“It is one of the oldest parts on the entire CSX system,” Gary Sease of CSX said.

A federal investigator says that the two locomotives were not derailed when 21 train cars went off the track. Eighteen of the 21 train cars that flew off the tracks in the derailment have already been removed from the scene.

Southworth also said the engineer-in-training was in control and traveling at the authorized speed of 25 miles per hour. Nine NTSB investigators are sifting through the wreckage. But the area is both tricky and dangerous.

Early Tuesday morning, the massive train traveling to Baltimore went off the tracks near the narrow Main Street bridge. Twenty-one cars overturned, dumping coal into the parking lot and street below.

Two teenagers– 19-year-old college students Elizabeth Nass and Rose Mayr– were on that train bridge with their backs to the side of the train as it passed a few feet behind them.

Their bodies were found buried in coal after the 9,000 ton-CSX train jumped the tracks. Federal investigators won’t say whether the presence of two teenagers sitting near the tracks was a factor in the derailment. They both died at the scene.

Just steps away from the center of the investigation, there’s a memorial to the lives that were lost.

The three crew members on board the train say they heard nothing unusual until the automatic emergency brake went off.

“There was a break in the air line– whether its brake pipe or an air hose or what, I’m not sure at this time. But when that happens, events start happening very fast. The reaction then from the system is to go towards derailment,” Southworth said.

The cleanup is a lengthy process. State environmental officials are investigating the impact of coal that spilled into the Patapsco River. Parts of the historic district have been off-limits since the derailment just after midnight on Tuesday.

The investigation has downtown Ellicott City at a standstill, its main artery shut down.

“I want this place open yesterday, so as soon as possible, we’re going to get back to work,” Ken Ulman, the Howard County executive, said.

Interview With Howard Co. Executive Ken Ulman:

Work that continues bucket by bucket.

CSX plans to continue operations on the train tracks after midnight on Wednesday. But Main Street will remain closed as crews continue cleaning up the coal.

The NTSB is reviewing footage captured from a camera at the front of the train and information from event recorders at their headquarters in Washington, D.C.

Several lawmakers are calling on CSX to take a closer look at safety.

“CSX must get to the bottom of what went wrong and outline what steps they are taking to ensure it will never happen again,” Sen. Barbara Mikulski said.

“Our focus right now is on this line and is on this investigation,” Gary Sease of CSX said.

The three CSX workers on board that train were an engineer, an engineer-in-training and a conductor. CSX hopes to have the tracks back up and running as early as Thursday.